A house once so famous that its creator grumbled "I keep an inn at the sign of the gothic castle" is almost ready to fling open its doors to paying guests again.

Despite a £9m restoration, Horace Walpole's extraordinary Strawberry Hill still has a few things missing; chairs, tables, beds, lamps, the comb of Saint Bertha, Cardinal Wolsey's hat and the golden armour of Francis I. Blame Lord Waldegrave, who hangs his head in shame on the topic. "Despite all his self-deprecating jokes, Walpole fully realised the importance of what he had created here, and he ensured a transmission mechanism - unfortunately that transmission mechanism was my family," the former Conservative health secretary said.

Walpole – gossip, aesthete, author and son of the first prime minister – never married and had no children. His sexuality was a matter for debate then and since.

He created a "little plaything house" – a fantasy of the middle ages with fireplaces, bookcases and ceilings modelled on the tombs of saints and kings – from a modest Georgian red brick cottage within view of the Thames, near Twickenham in west London.

It became the first gothic revival house in the world, inspiring countless imitations and giving the world the term "Strawberry Hill gothic". He wrote a guidebook and list of rules – which didn't prevent one tourist snapping off the beak of a Roman eagle as a souvenir – for the hordes who clamoured for tickets to tour its wonders, including a royal bedroom in which nobody ever slept and a ceiling painted with the crests of invented ancestors.

Michael Snodin, chair of the trust reopening it to the public as a museum, said without Horace Walpole, Augustus Pugin's Palace of Westminster would never have been built – although Pugin "utterly despised" Walpole's lack of seriousness.

When Walpole died in 1797 he left the house to his niece and it soon passed by marriage to the Waldegraves – and two successive heirs described sadly by Lord Waldegrave as "absolute gangsters".

"They were just awful, worthy villains for a Dickens novels, drunks, gamblers, if there had been drugs and rock'n'roll then they would have done that too." John died young having raced through much of the family fortune, and his brother George then did for the rest.

In 1841 he beat up a policeman in Kingston while drunk, and the Twickenham magistrate jailed him for six months. George took his revenge on Twickenham, and cleared some of his debts, by holding the Great Sale which occupied the Strawberry Hill lawns for a month in 1842. Special excursion coaches were run from central London and the house was stripped of everything down to the door knobs.

The same woman, Frances Braham, had married both the rakehell Waldegraves. She went on to marry twice more and became seriously rich, allowing her to restore and extend the house. She was such a renowned political society hostess that Disraeli – who knew another brilliant social climber when he saw one – called her "the real leader of the opposition."

Lord Waldegrave continues the family penance as a trustee and donor.

By the turn of the millennium the Grade I listed house was so decayed that the World Monuments Fund included it among the 100 most endangered heritage sites of the world, sparking the formation of the trust, and restoration grants from both English Heritage and the Heritage Lottery Fund.

It will reopen on 2 October with a tea room for the first time, but public interest is such that the first couple of months are already booked out.

Replica furniture will be recreated for some of the empty rooms, but Snodin still hopes original contents may come back on loan.

However the house has acquired some new artefacts over the years of work. Master carpenter Phillip Auciello found a 1760s chisel behind the panelling of one room and a late 19th century chisel under the floor of another - but has lost a new hammer, which he believes may now rest behind the plaster in the new ceiling of the library.